Two-photon processes based on quantum commutators Fratini, Filippo Safari, Laleh Amaro, Pedro Santos, José We developed a method to calculate two-photon processes in quantum mechanics that replaces the infinite summation over the intermediate states by a perturbation expansion. This latter consists of a series of commutators that involve position, momentum, and Hamiltonian quantum operators. We analyzed several single- and many-particle cases for which a closed-form solution to the perturbation expansion exists, as well as more complicated cases for which a solution is found by convergence. Throughout the article, Rayleigh and Raman scattering are taken as examples of two-photon processes. The present method provides a clear distinction between the Thomson scattering, regarded as classical scattering, and quantum contributions. Such a distinction lets us derive general results concerning light scattering. Finally, possible extensions to the developed formalism are discussed. American Physical Society 2018 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/294 Fratini F, Safari L, Amaro P, Santos J. Two-photon processes based on quantum commutators. <i>Physical Review A - Atomic, Molecular, and Optical Physics</i>. 2018;97(4). doi:<a href="https://doi.org/10.1103/PhysRevA.97.043842">10.1103/PhysRevA.97.043842</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1103/PhysRevA.97.043842 info:eu-repo/semantics/altIdentifier/wos/000430296800008 info:eu-repo/semantics/altIdentifier/arxiv/1801.06892 info:eu-repo/semantics/openAccess